Electrophotographic print architectures are used in processing devices such as copy machines and laser printers. Generally, a laser generates a positively charged negative image on a photoconductive drum. A negatively charged toner is attracted to the positively charged negative image on the photoconductive drum. The toner from the negative image on the drum is attracted to a positively charged sheet of paper as the sheet is advanced by the photoconductive drum, generating a positive image on the sheet. The toner is then fused to the sheet using pressure, a combination of heat and pressure, or light, causing the toner in the positive image to permanently adhere to the sheet.
Devices employing various electrophotographic print architectures have been developed. Some devices utilize modular electrophotographic print architectures, which may include multiple sheet transport modules to print, fuse, or otherwise process one or more sheets of paper as they are advanced through the transport modules. The sheet transport modules may include one or more print modules, one or more pre-processing modules, and one or more post-processing modules. For example, certain monochrome print architectures may include one print module to deposit a single color toner (e.g., black) as a sheet of paper is transported. The pre-processing and post-processing transport modules may perform sheet inversion, sheet decurling, sheet charge neutralization, sheet registration, sensing sheet properties or sensing printed images on a sheet. Certain color devices may include multiple print modules with each print module depositing a different color toner and a fuser module fusing the color toners to the sheet. Thus, various sheet transport modules may exist in different electrophotographic print architectures.
Certain modular electrophotographic print architectures include sheet transport modules that utilize electrostatic escort belts to transport sheets of paper through the sheet transport modules. In such modular electrophotographic print architectures, a first sheet transport module electrostatically tacks an incoming sheet of paper to its escort belt and transports it through the first sheet transport module. As the sheet of paper exits the first sheet transport module, a second sheet transport module tacks the sheet of paper to its escort belt and transports it through the second sheet transport module. Although it is desired that the escort belts operate at the same surface velocity, the surface velocity of the escort belts may vary due to differences in mechanical tolerances and imperfections in the escort belt structure of each sheet transport module. For example, the imperfections may include escort belt thickness variations, drive roll and tension roll diameter variations, conicity, wobble, run-out, drive-train vibrations, torque ripple, as well as other imperfections. The surface velocity differences are also pervasive in other modular electrophotographic print architectures regardless of transport technology employed in the sheet transport modules.
Notwithstanding the transport technology employed in the modular electrophotographic print architectures, when a sheet of paper is transported by several sheet transport modules simultaneously—such as during transfer of sheets between the sheet transport modules—large pulling forces or buckling forces may accumulate in the sheet. These forces are undesirable as they may cause the sheet to tear, buckle or slip in the sheet transport modules. These forces may also cause vibration, such as during transfer of sheets between the sheet transport modules, which negatively affects image quality such as color-to-color registration in color devices, as well as smearing and banding in color and monochrome devices.